ejabberd.yml.example: Enable mod_muc_occupantid

Add mod_muc_occupantid to the list of modules enabled in the sample
configuration.  It's not necessarily obvious that it's required for
using certain modern features in group chat, and there's no downside in
activating this module.

ejabberd.yml.example

@@ -199,6 +199,7 @@ modules:
     default_room_options:
       mam: true
   mod_muc_admin: {}
+  mod_muc_occupantid: {}
   mod_offline:
     access_max_user_messages: max_user_offline_messages
   mod_ping: {}